This invention relates to an engine starter motor with a planetary speed reduction gear, and more particularly to improvements in internal gears of the planetary seed reduction gear for an engine starter motor.
One example of a conventional coaxial engine starter motor including a planetary speed reduction gear disclosed in Japanese Utility Model Laid-Open No. 63-71474 (Japanese U. M. Application No. 61-166198), for example, is illustrated in FIG. 1. The coaxial engine starter motor shown in FIG. 1 comprises a d.c. electric motor 1 having an armature rotary shaft 1a, an output rotary shaft 3 axially slidably supported and disposed at one end of the d.c. motor 1, to which the rotation of the armature rotary shaft 1a is transmitted through the drive force transmission mechanism including a planetary speed reduction gear 2, a pinion 4 disposed at the front end of the output rotary shaft 3, and a solenoid switch 5 disposed at the other axial end of the d.c. electric motor 1 for pushing and sliding the output rotary shaft 3 by a plunger rod 5a at the same time as the d.c. electric motor 1 is energized, the armature rotary shaft 1a of the d.c. electric motor 1 and the plunger rod 5a of the solenoid switch 5 are coaxially aligned.
In such the conventional coaxial engine starter, the front frame 6 housing the solenoid switch 5, the d.c. motor 1 and the drive force transmission mechanism are assembled and connected by two through bolts 7. That is, each of the through bolts 7 is inserted into a hole formed in a flange portion 5c formed around the front end portion of a case 5b of the solenoid switch 5, passes outside of the outer periphery of a yoke 1b of the d.c. motor 1 and the outer periphery of the internal gear 8 constituting the planetary speed reduction gear 2 and thread-engaged at its threaded end into the threaded hole of the front frame 6, whereby the case 5b of the solenoid switch 5 and the front frame 6 are pulled to each other to securely press the end of the yoke of the d.c. motor 1, thereby to connect the above three machine components to each other to provide a coaxial engine starter integrally assembled into one piece.
As above described, the through bolts 7 for connecting the solenoid switch 5, the d.c. motor 1 and the front frame 6 into one piece pass around the outer periphery of the internal gear 8 of the planetary speed reduction gear 2. Therefore, the dimension of the internal gear 8 is inevitably limitted by the through bolts 7 as shown in FIG. 5. More particularly, as shown in FIG. 2 deddendum circle and the pitch circle of the internal gear 8 are determined by taking into consideration the mechanically necessary thickness of the rim thickness 1 between the bottom 8b and the outer peripheral surface 8c of the teeth 8a when the through bolt 7 is disposed adjacent to the outer periphery, so that the internal gear 8 cannot be made larger beyond this limitation.
Further, the conventional starter motor may include an internal gear integrally formed with the front frame 6. Even in this case, the size of the internal gear or the diameter of the dedendum circle or the pitch circle of the internal gear is similarly limited by the position of the threaded holes for receiving the through bolts which are formed in the axial direction from the end face of the front frame 6.
With such the limitation of the dimension of the internal gear 8 in the planetary speed reduction gear 2, the number of teeth is also limited, so that the degree of freedom in the speed reduction ratio design is significantly and disadvantageously decreased. While this problem can be solved by locating the through bolts as for outside as possible, this arrangement makes the front frame having the threaded holes for receiving the bolts larger only for that purpose, resulting in a large and heavy front frame, and hence hindering the development of a small-sized, light-weight starter motor.